Master Gunsmith

Master Gunsmith 
You may design your own weapons, they are made using a point buy system as follows.
Your Wisdom or Intelligence modifier + Proficiency Bonus added together to produce your points pool.

To make your own guns you need several things, a foundry to cast metal parts and smith parts.
Each upgrade was probably not achieved on the first try, so each upgrade is 100gp and takes a full day to build and install. 

Base damage die is 1D8

*   +3 Rounds in the magazine per point spent
*     1 additional damage type per point spent
*   +1 additional to hit and base damage per point spent.
*     Cone blast effect for 2 Points
*     Mortar Effect for 2 Points
*     Oversized Caliber for 3 Points (additional Damage dice)
*     Double Range is 3 Points and makes the feature ‘Scope/Iron sights’
*     Extra Barrel is 2 Points

Scope/Iron Sights
You can create a scope which doubles the range of the firearm, however any shot taken under 30ft is rolled at disadvantage.
For 1 action you may remove the scope and switch to iron sights or for one action you can put it back.